It is currently estimated that about 200,000,000 used tires are produced annually. After the useful lifetime of these tires has expired, they are commonly dumped in waste disposal sites, but frequently are discarded on vacant land and in lakes and rivers because the used tires have no value and, in fact, cost money to dispose of properly.
In years past, it was possible for waste disposal sites and reclaiming plants to burn the tires and recover the metal or to dump them at a landfill. However, with increasing public awareness of ecology and the state of the environment, both the State Environmental Protection Agencies and the U.S. Government Environmental Protection Agency, have sought to eliminate this form of air and land pollution.
The most common traditional method for handling waste materials has been to simply dump the waste on designated land areas thereby creating large and often hazardous dumps in which fires are frequent and runoff into water sources is routine. Not only are these dumps eyesores, they are environmental hazards to both the air and water. It is of no little consequence that while dumps or landfills have often been the cheapest short term way of handling waste material, the land which can be used for this purpose is rapidly running out. This is particularly true in the highly urbanized areas of the industrialized countries. This traditional method of disposing of waste does not allow for the recovery of materials.
A particular problem is found in the disposal of rubber based products, such as automotive tires, hoses and belts, all of which are comprised of natural or synthetic rubber reinforced with other materials such as metal belts and fibrous cords. These products have very little use after they have performed their original and/or primary task and therefore have generally discarded. It is acknowledged that a certain number of tires are used to build retaining walls, guards protecting boating and similar things where resistance to weathering is desirable. However, a far greater number of tires, belts and hoses are simply discarded to become eyesores and breeding grounds for insects and other pests. Burying is particularly ineffective since these materials resist decomposition and tend to work their way to the surface.
There have been many proposals for handling waste materials so as to recover the component products. Some of these included burning off the unwanted material to get at the fire resistant residues or metal substructure. While this may seem to be an answer, it ignores the fact that much useful material would be consumed in burning, that the incineration process itself would not be energy efficient and could release hazardous byproducts into the atmosphere. It also ignores the problem of what to do with the ashes.
Other approaches to material recovery have included rendering the waste material into smaller pieces and then forcibly removing the desirable components. This may be a very difficult task when one considers recovery of material from something such as a tire, which is produced to withstand much abuse without loss of function. Even rendering a tire into smaller pieces would still not enhance the recovery of the metal, fiber and rubber products thereof. Besides a great amount of energy would be consumed in rendering the tire into the smaller pieces.
Still other approaches have involved the use of chemicals to break down the materials into their components. However, these methods create chemical sludges and residues which are not only a nuisance, if not impossible to dispose of, but some chemical treatments are dangerous to both human life and the environment.
Still additional approaches have involved the extensive use of cryogenics to lower the temperature of the product to below the glass-transition temperature of the components thereof. The product at this lowered temperature is then crushed sufficiently to cause the components to release sufficiently to effect separation. However, this in an energy intensive process.
The application of ultrasonic waves to the process of devulcanizing rubber is a most recent field. In fact, traditional thinking in the field has indicated that rubber is vulcanized by ultrasonics rather than devulcanized. Okada and Hirano published in Meiji Gomu Kasei, 9(1), 14-21 (1987) that the ultrasonic vulcanization of rubber was achieved, and that process demonstrated in the laboratory.
Additionally, the use of ultrasound to activate rubber-based adhesives was discussed in Kauch. Rezina, (5), 31-2 (1983) where the bonding of rubber strips was described. The dynamic strength of the bonded rubber strips was found to increase with increasing ultrasonic activation time.
Again, the ultrasonic welding of composite polymers was discussed in Svar. Proizvod., (7), 42-3 (1982) where the ultrasonic welding of carbon black-filled rubber was determined to proceed via crosslinking between rubber and the carbon black molecules.
The vulcanization of rubber and crosslinking of polymers by ultrasound is elaborated in DE 2,216,594 published Oct. 26, 1972, based on Japanese priority document JP 71-20736 dated Apr. 6, 1971 wherein ethylene-propylene rubber or polybutadiene rubber-natural rubber mixtures; or polymers, e.g. polyethylene containing vulcanization agents or crosslinking agents were vulcanized or crosslinked, respectively, by ultrasonic waves (500 kHz) in a bath containing cold water or silicone oil.
The application of continuous ultra-high frequency vulcanization to high polarity rubbers, such as chloroprene rubber or butadiene-nitrile rubber is described, Rubber World, 162(2), 59-63 (1970).
The only application of ultrasonic waves in a devulcanization mode is described in JP 62121741 (1987). A batch process was described wherein vulcanized rubber was reclaimed by devulcanization by 10 kHz to 1 MHz ultrasonic wave irradiation. The batch process required 20 minutes using 50 kHz ultrasonic wave at 500 watts. The process was found to break carbon-sulfur bonds and sulfur-sulfur bonds, but not carbon-carbon bonds.